This details an entire potential storyline for the game as of August 22nd 1997 and is available as a pdf from here.

The Ultima connection appears to have been dropped in favour of another fantasy world and the game is far less dungeon based than UW1 & 2. The document is only a brief outline and there is precious little about gameplay unfortunately. I find it hard to form an opinion of how Underworld 3 might have turned out based on this alone but the scale of some of the events was certainly ambitious for 3D engines of the time and the potential was there. It’s hard not to think that this would have been in direct competition with Ultima 9 which could be one reason why it never happened.​

The Ultima connection appears to have been dropped in favour of another fantasy world and the game is far less dungeon based than UW1 & 2. The document is only a brief outline and there is precious little about gameplay unfortunately. I find it hard to form an opinion of how Underworld 3 might have turned out based on this alone but the scale of some of the events was certainly ambitious for 3D engines of the time and the potential was there. It’s hard not to think that this would have been in direct competition with Ultima 9 which could be one reason why it never happened.

The game would have begun in a way not unlike Ultima 9 did: your character (presumably the Avatar) would have been enjoying a quiet retreat at a lakeside cottage when, quite suddenly, you are drawn into a moongate and whisked off to the world of Jaal, which was to be filled with dungeons, underwater cities, pirates…and, of course, the usual evil and intrigue that the Avatar is so expert at addressing and dealing with. Much of the game would, seemingly, have centered around traveling between destinations on a pirate ship which I can only surmise was named for Janis Joplin. The game would ultimately have ended on a bit of a cliffhanger, implying that there was some intention of seeing the Underworld games continue as a series.​